It is known that colored photographic images can be produced by chromogenic development, i.e. by development of silver halide emulsion layers which have been exposed to form an image with suitable dye-producing developer substances (so-called color developers) in the presence of suitable color couplers, the developer oxidation product formed in accordance with the silver image reacting with the color coupler to form a dye image. The color couplers used are normally aromatic compounds containing primary amino groups, more especially of the p-phenylenediamine type.
In practice, color couplers and the dyes produced therefrom by chromogenic development have to satisfy a number of requirements. Thus, the rate at which the color couplers couple with the oxidation product of the color developer should be as high as possible. The color couplers and the dyes obtained therefrom should show adequate stability to light, elevated temperature and moisture. This applies both to fresh material and also to processed material. For example, the residual coupler still present in the white parts of the processed material should not turn yellow. In addition, the dyes should show adequate stability to gaseous reducing or oxidizing agents. In addition, they should be anchored in non-diffusing form in the image layer and should be deposited in fine-grained form during the chromogenic development process. The mechanical properties of the layers should not be adversely affected by the color couplers. Finally, the dyes formed from the color couplers during the chromogenic development process should show a favorable absorption curve with a maximum which corresponds to the color of the particular component image required and minimal secondary absorptions. Thus, in the ideal case, a cyan dye should absorb red light almost completely while allowing most green and blue light through. In addition, the absorption maxima of the dyes both in color reversal films and in color negative films should substantially correspond to the sensitization maxima of the color paper materials used for copying.
Compounds derived from phenol or .alpha.-naphthol are generally used as cyan couplers, i.e. as color couplers suitable for producing the cyan component image. Although naphtholic cyan couplers give dyes with substantially ideal absorption for color negative films (absorption maximum at approximately 700 nm), they show very poor dark-fading behavior, i.e. the cyan component image formed during development fades to a considerable extent in the event of longterm storage or in an accelerated test at elevated temperature, resulting in reddening of the copy or in a density-dependent color tinge.
Naphtholic cyan couplers containing a heterocyclic substituent in the 2-position are also known from a number of patent specifications (DDR 5567, DDR 5906, U.S. Pat. Nos. 2,373,821, 2,530,349, 2,545,687, 2,545,307, DE-B-1 009 923, FR-A-878 943).
These compounds are also characterized by good absorption of the dyes produced therefrom with a maximum at approximately 700 nm and, in particular, by high permeability in the green spectral region.
Benzthiazoles, benzoxazoles and benzimidazoles are mentioned as examples of heterocyclic substituents in the 2-position.
In addition, other photographically important groups, such as for example solubilizing groups and/or ballast groups, may be arranged in the heterocyclic radical.
The disadvantage of all these compounds lies in the poor re-oxidizability of the leuco dyes intermediately formed, particularly in spent bleaching or bleach-fixing baths or in atypically prepared bleaching or bleach-fixing baths. This results in a considerable reduction in the maximum obtainable density, for example when the bleaching time is shortened, when the pH is changed or when developers, are carried over.